Overcurrent determining apparatus and drive unit using the same

ABSTRACT

In an overcurrent determining apparatus, a temperature obtainer obtains a temperature parameter indicative of a temperature of a switching element as a temperature measurement value. A determination voltage has a first voltage value when the temperature measurement value is a first temperature. A setter sets the determination voltage to a second voltage value upon determining that the temperature measurement value is a second temperature higher than the first temperature. The second voltage value is lower than the first voltage value and higher than a value of a Miller voltage of the switching element at the second temperature.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is based on and claims the benefit of priority fromJapanese Patent Application 2017-214168 filed on Nov. 6, 2017, thedisclosure of which is incorporated in its entirety herein by reference.

TECHNICAL FIELD

The present disclosure relates to overcurrent determining apparatusesfor determining whether an overcurrent is flowing between a pair of mainterminals of a switching element, and also relates to drive units usingat least one of the overcurrent determining apparatuses.

BACKGROUND

For protecting a switching element, such as an IGBT, against anovercurrent, conventional overcurrent determining apparatuses areconfigured to

(1) Determine whether a voltage at an open-close control terminal, i.e.an on-off control terminal, of the switching element has exceeded apredetermined determination voltage within a predetermined determinationperiod

(2) Determine that an overcurrent is flowing between the pair of mainterminals of the switching element upon determining that the voltage atthe open-close control terminal of the switching element has exceededthe predetermined determination voltage within the predetermineddetermination period

An example of these conventional overcurrent determining apparatuses isdisclosed in Japanese Patent Application Publication No. 2015-19554. Thedetermination voltage is set to be higher than a predetermined Millervoltage of the switching element and to be lower than an upper limit ofthe voltage at the open-close control terminal. In addition, thedetermination period is set to be within a reference period for which,if no overcurrent flows between the pair of main terminals of theswitching element, the voltage at the open-close terminal of theswitching element has reached a determination voltage since it wasclosed.

SUMMARY

Users have an increasing demand for such an overcurrent determiningapparatus to determine whether an overcurrent is flowing through aswitching element much earlier. This is because the delay in theovercurrent determination may increase the integral of the overcurrenthaving flowed between the main terminals of the switching element untilcompletion of the overcurrent determination.

In view of the circumstances set forth above, a first aspect of thepresent disclosure seeks to provide overcurrent determining apparatuses,each of which is capable of addressing the problem set forth above.

Specifically, a second aspect of the present disclosure aims to providesuch overcurrent determining apparatuses, each of which is capable ofdetermining whether an overcurrent is flowing through a switchingelement much earlier.

A third aspect of the present disclosure aims to provide drive units,each of which includes at least one of such overcurrent determiningapparatuses according to the second aspect.

According to a first exemplary aspect of the present disclosure, thereis provided an overcurrent determining apparatus applicable to aswitching circuit. The switching circuit includes a switching elementhaving first and second main terminals and an open-close controlterminal, and a charging unit configured to supply electrical charge tothe open-close control terminal of the switching element to therebycharge the open-close control terminal. The overcurrent deter miningapparatus is configured to execute an overcurrent determination routineto determine whether an overcurrent is flowing through the pair of firstand second main terminals of the switching element based on whether apredetermined condition that a terminal voltage at the open-closecontrol terminal of the switching element is higher than a determinationvoltage is satisfied. The overcurrent determining apparatus includes atemperature obtainer configured to obtain a temperature parameterindicative of a temperature of the switching element as a temperaturemeasurement value. The determination voltage has a first voltage valuewhen the temperature measurement value is a first temperature. Theovercurrent determining apparatus includes a setter configured to setthe determination voltage to a second voltage value upon determiningthat the temperature measurement value is a second temperature higherthan the first temperature. The second voltage value is lower than thefirst voltage value and higher than a value of a Miller voltage of theswitching element at the second temperature.

The overcurrent determining apparatus according to the first exemplaryaspect is configured to set the determination voltage to the secondvoltage value upon determining that the temperature measurement value isthe predetermined second temperature which is higher than the firsttemperature. The second voltage value is lower than the first voltagevalue and higher than the value of the Miller voltage of the switchingelement at the second temperature. This enables a determination periodneeded to determine whether an overcurrent is flowing through theswitching element using the second voltage value at the secondtemperature to be shorter than a determination period needed todetermine whether an overcurrent is flowing through the switchingelement using the first voltage value at the first temperature.

In contrast, if the determination voltage were set to a constant valueindependently of the temperature of the switching element, a value ofthe determination voltage might be set to be higher based on the valueof the Miller voltage at the first temperature. This might result in thedetermination period being longer independently of the temperaturemeasurement value of the switching element.

That is, because the above configuration of the overcurrent determiningapparatus according to the first exemplary aspect makes shorter thedetermination period, making it possible to determine whether anovercurrent is flowing through the switching element earlier.

According to a second exemplary aspect of the present disclosure, thereis provided an overcurrent determining apparatus applicable to aswitching circuit. The switching circuit includes at least first andsecond switching elements parallely connected to each other. Each of theat least first and second switching elements has first and second mainterminals and an open-close control terminal. The switching circuitincludes a charging unit configured to supply electrical charge to theopen-close control terminal of each of the at least first and secondswitching elements to thereby charge the open-close control terminal.The overcurrent determining apparatus is configured to execute anovercurrent determination routine to determine whether an overcurrent isflowing through the pair of first and second main terminals of each ofthe at least first and second switching elements based on whether apredetermined condition that a terminal voltage at the open-closecontrol terminal of the corresponding one of the at least first andsecond switching elements is higher than a determination voltage issatisfied.

The overcurrent determining apparatus includes a temperature obtainerconfigured to obtain at least first and second temperature parametersrespectively indicative of temperatures of the at least first and secondswitching elements. The at least first and second temperature parametersare respectively referred to as at least first and second temperaturemeasurement values.

The overcurrent determining apparatus includes a setter havingcorrelation data indicative of the relationship between thedetermination voltage and each of the at least first and secondtemperature measurement values such that, the higher each of the atleast first and second temperature measurement values is, the lower thedetermination voltage is.

The setter is configured to commonly set a value of the determinationvoltage for each of the at least first and second switching elements toa selected value on the correlation data, the selected valuecorresponding to the lowest value in the at least first and secondtemperature measurement values.

The overcurrent determining apparatus according to the second exemplaryaspect includes the setter having the correlation data indicative of therelationship between the determination voltage and each of the at leastfirst and second temperature measurement values such that, the highereach of the at least first and second temperature measurement values is,the lower the determination voltage is.

The setter is configured to commonly set a value of the determinationvoltage for each of the at least first and second switching elements toa selected value on the correlation data, the selected valuecorresponding to the lowest value in the at least first and secondtemperature measurement values.

This configuration enables the determination period for each of the atleast first and second switching elements to be shorter, making itpossible to determine whether an overcurrent is flowing through at leastone of the at least first and second switching elements earlier.

In particular, the overcurrent determining apparatus according to thesecond exemplary aspect is configured such that a value of thedetermination voltage is set to be a highest value corresponding to thelowest value in the at least first and second temperature measurementvalues. If a value of the determination voltage were set to be a lowervoltage value corresponding to a temperature measurement value exceptfor the lowest value in the at least first and second temperaturemeasurement values, the value of the determination voltage might belower than the value of the Miller voltage of a selected one of the atleast first and second switching elements corresponding to the lowestvalue. This therefore might result in the overcurrent determiningapparatus erroneously determining that an overcurrent is flowing throughthe selected one of the at least first and second switching elementsalthough there is no actual overcurrent flowing through the selected oneof the at least first and second switching elements.

In contrast, the above configuration of the overcurrent determiningapparatus according to the second exemplary aspect enables a value ofthe determination voltage for each of the at least first and secondswitching elements to be set to the selected value corresponding to thelowest value in the at least first and second temperature measurementvalues. This configuration therefore prevents the overcurrentdetermining apparatus from erroneously determining that an overcurrentis flowing through the selected one of the at least first and secondswitching elements.

According to a third exemplary aspect of the present disclosure, thereis provided a drive unit including a switching circuit. The switchingcircuit includes a switching element having first and second mainterminals and an open-close control terminal, and a charging unitconfigured to supply electrical charge to the open-close controlterminal of the switching element to thereby charge the open-closecontrol terminal. The drive unit includes a drive controller configuredto

(1) Execute an overcurrent determination routine to determine whether anovercurrent is flowing through the pair of first and second mainterminals of the switching element based on whether a predeterminedcondition that a terminal voltage at the open-close control terminal ofthe switching element is higher than a determination voltage issatisfied

(2) Obtain a temperature parameter indicative of a temperature of theswitching element as a temperature measurement value, the determinationvoltage having a first voltage value when the temperature measurementvalue is a first temperature

(3) Set the determination voltage to a second voltage value upondetermining that the temperature measurement value is a secondtemperature higher than the first temperature, the second voltage valuebeing lower than the first voltage value and higher than a value of aMiller voltage of the switching element at the second temperature.

The drive unit according to the third exemplary aspect obtains the samebenefit obtained by the overcurrent determining apparatus according tothe first exemplary aspect.

BRIEF DESCRIPTION OF THE DRAWINGS

Other aspects of the present disclosure will become apparent from thefollowing description of embodiments with reference to the accompanyingdrawings in which:

FIG. 1 is a circuit diagram schematically illustrating an overallconfiguration of a control system for a motor-generator according to thefirst embodiment of the present disclosure;

FIG. 2 is a circuit diagram schematically illustrating an example of thestructure of each drive unit of the control system illustrated in FIG.1;

FIG. 3 is a flowchart schematically illustrating an example of anovercurrent determination routine carried out by each drive unit of thecontrol system illustrated in FIG. 1;

FIG. 4 is a graph schematically illustrating

(1) An example of the relationships between the temperature of aswitching element and a gate determination voltage

(2) An example of the relationships between the temperature of theswitching element and a Miller voltage according to the firstembodiment;

FIGS. 5A to 5D are a joint timing chart schematically illustrating anexample of the overcurrent determination routine executed by the drivecontroller if no overcurrent is flowing through a target switchingelement in a normal condition;

FIGS. 6A to 6D are a joint timing chart schematically illustrating anexample of the overcurrent determination routine executed by the drivecontroller if an overcurrent is flowing through the target switchingelement in an abnormal condition;

FIG. 7 is a circuit diagram schematically illustrating an example of thestructure of each drive unit according to the second embodiment of thepresent disclosure;

FIG. 8 is a flowchart schematically illustrating an example of anovercurrent determination routine carried out by each drive unitaccording to the second embodiment;

FIG. 9 is a circuit diagram schematically illustrating an example of thestructure of each drive unit according to the third embodiment of thepresent disclosure;

FIG. 10 is a flowchart schematically illustrating an example of anovercurrent determination routine carried out by each drive unitaccording to the third embodiment;

FIG. 11 is a graph schematically illustrating

(1) An example of the relationships between the temperature of aswitching element and a gate determination voltage

(2) An example of the relationships between the temperature of theswitching element and a Miller voltage according to the thirdembodiment; and

FIG. 12 is a circuit diagram schematically illustrating an example ofthe structure of each drive unit according to the fourth embodiment ofthe present disclosure.

DETAILED DESCRIPTION OF EMBODIMENT

The following describes embodiments of the present disclosure withreference to the accompanying drawings. In the embodiments, like partsbetween the embodiments, to which like reference characters areassigned, are omitted or simplified in description to avoid redundantdescription.

First Embodiment

Referring to FIG. 1, there is illustrated a three-phase motor-generatoras an example of rotating machines, referred to simply as a“motor-generator” 10, installed in, for example, a hybrid vehicle or anelectrical vehicle as its main engine according to the first embodiment.The motor-generator 10 is mechanically coupled to driving wheels (notshown) of the hybrid vehicle or the electrical vehicle.

For example, as the motor-generator 10, a brushless DC motor, i.e. athree-phase permanent magnet SM (Synchronous Motor), is used.

The motor-generator 10 is comprised of, for example, a rotor including amagnetic field and a stator including three-phase windings, i.e. U-, V-,and W-phase windings. The rotor of the motor-generator 10, whichincludes permanent magnets, is rotated based on magnetic interactionbetween the magnetic field of the permanent magnets of the rotor and arotating field generated by the three-phase windings when thethree-phase windings are energized. For example, the three-phasewindings (U-, V-, and W-phase windings) each have a first end connectedto a common junction (neutral point) in, for example, astar-configuration.

In FIG. 1, there is also illustrated a control system 100 forcontrolling the motor-generator 10. The control system 100 is equippedwith an inverter 11, a high-voltage battery 12, drive units, i.e.drivers, DU, a controller 14, a low-voltage battery 16, and an interface18.

To the motor-generator 10, the high-voltage battery 12 is electricallyconnected via the inverter 11. The high-voltage battery 12 has aterminal voltage, which is equal to or higher than 100 V, thereacross. Alithium-ion storage battery or a nickel-hydrogen storage battery can beused as the high-voltage battery 12.

The inverter 11 is designed as a three-phase inverter. The inverter 11includes three pairs of series-connected high- and low-side (upper- andlower-arm) switching elements SUp and Sun, SVp and SVn, and SWp and SWn.The inverter 11 also includes flywheel diodes DUp and DUn, DVp and DVn,and DWp and DWn electrically connected in antiparallel to thecorresponding switching elements SUp and SUn, SVp and SVn, and SWp andSWn, respectively.

In the first embodiment, as each switching element S*# (*=U, V, W, #=p,n), a voltage-controlled semiconductor switching element, such as anIGBT, is used.

When power MOSFETs are used as the switching elements S*# (*=U, V, W, #=p, n), intrinsic diodes of the power MOSFETs can be used as theflywheel diodes, thus eliminating the flywheel diodes.

Each of the switching elements S*# has a pair of an output terminal,i.e. the emitter of the IGBT, and an input terminal, i.e. the collectorof the IGBT, which corresponds to a pair of main terminals of thecorresponding switching element. In particular, the collector of eachswitching element (IGBT) corresponds to a first main terminal in thepair of main terminals thereof, and the emitter of each switchingelement (IGBT) corresponds to a second main terminal in the pair of mainterminals thereof.

Adjusting a voltage at the open-close control terminal of each switchingelement S*#, such as the gate of each IGBT, to be higher than apredetermined threshold voltage Vth for the corresponding switchingelement S*# enables the corresponding switching element S*# to beswitched from an off state (open state) to an on state (closed state).The voltage at the open-close control terminal of each switching elementS*#, such as the gate of each IGBT, will be referred to as a gatevoltage Vge. The threshold voltage Vth for each switching element S*#represents a value of the gate voltage Vge at which the correspondingswitching element S*# is switched from the off state to the on state. Indetail, the threshold voltage Vth for the gate voltage Vge of eachswitching element appears when a reference current having a referencelevel of, for example, 1 mA is flowing through the correspondingswitching element.

The three pairs of switching elements are parallelly connected to eachother in bridge configuration. A connecting point through which each ofthe switching elements S*p is connected to a corresponding one of theelements S*n in series is connected to a busbar extending from thesecond terminal of a corresponding one of the U-phase winding, V-phasewinding, and W-phase winding. A first end of the series-connectedswitching elements of each of the three pairs, such as the collector ofthe corresponding high-side switching element, is connected to thepositive terminal of the high-voltage battery 12 via a positive DC line.The opposite second end of the series-connected switching elements ofeach of the three pairs, such as the emitter of the correspondinglow-side switching element, is connected to the negative terminal of thehigh-voltage battery 12 via a negative DC line.

The inverter 11 also includes temperature-sensitive sensors TUp and TUn,TVp and TVn, and TWp and TWn located to be close to the correspondingswitching elements SUp and SUn, SVp and SVn, and SWp and SWn,respectively. Each temperature-sensitive sensor T*# is configured tomeasure a temperature of the corresponding switching element S*#. Thatis, each semiconductor switch Ze is comprised of the correspondingswitching element S*#, the corresponding flywheel diode D*#, and thecorresponding temperature-sensitive sensor T*# packaged therein. Atemperature-sensitive diode or a thermistor can be used as eachtemperature-sensitive sensor T*#.

For example, the controller 14 is mainly comprised of a microprocessorand a non-transitory computer readable storage medium. The controller 14operates on a power-supply voltage, lower than the terminal voltageacross the high-voltage battery 12, supplied from the low-voltagebattery 16. Thus, the controller 14 and the low-voltage battery 16constitute a low voltage system. In contrast, the motor-generator 10,the inverter 11, and the high-voltage battery 12 constitute a highvoltage system.

The controller 14 is designed to individually drive the switchingelements SUp, SUn, SVp, SVn, SWp, and SWn of the inverter 11 to therebyadjust a controlled variable of the motor-generator 10, such as anoutput torque of the motor-generator 10, to a commanded value.

Specifically, the controller 14 is designed to individually generate andsend drive signals gUp, gUn, gVp, gVn, gWp, and gWn to the drive unitsDU provided for the respective switching elements SUp, SUn, SVp, SVn,SWp, and SWn. This causes the respective drive units DU to individuallyturn on or off the respective switching elements SUp, SUn, SVp, SVn,SWp, and SWn. The individual turn-on or off of the respective switchingelements SUp, SUn, SVp, SVn, SWp, and SWn convert the output DC voltageacross the capacitor C into an AC voltage, and supply the AC voltage tothe motor-generator 10.

Each of the drive signals g*# has a predetermined duty cycle, i.e. apredetermined ratio of on duration to the total duration of eachswitching cycle for a corresponding one of the switching elements S*#(see FIG. 1).

Specifically, the controller 14 is designed to complementarily turn onthe high- and low-side switching elements S*# for each leg (phase) viathe corresponding drive units DU according to the corresponding drivesignals g*#. In other words, the controller 14 is designed toalternately turn on the high-side switching element S*p of one leg(phase) and the low-side switching element S*n of the same leg (phase).This drive alternately closes the conductive path between the collectorand emitter of the high-side switching element S*p of one leg and theconductive path between the collector and emitter of the low-sideswitching element S*n of the same leg.

The interface 18 is capable of electrically isolating the high voltagesystem and the low voltage system. Specifically, the interface 18includes insulation members, such as photocouplers, provided for therespective switching elements S*# of the inverter 11. Each of thephotocouplers is comprised of a photodiode and a phototransistor. Thephotocouplers are configured to enable communications between the highand low voltage systems while establishing electrical isolationtherebetween. Specifically, each of the photocouplers is configured toenable the controller 14 to control a corresponding one of the switchingelements S*# while establishing electrical isolation between thecontroller 14 and a corresponding one of the switching elements S*#.

Next, the following describes an example of the structure of each driveunit DU provided for a corresponding one switching element S*# withreference to FIG. 2.

Referring to FIG. 2, the drive unit DU is comprised of a drive IC 20,which is, for example, configured as a single chip semiconductor IC, aconstant voltage power source 22, a constant current power source 24, adischarging resistor 28, a soft turnoff resistor 38, and a senseresistor 42.

The drive IC 20 has first to ninth terminals T1 to T9, and the constantcurrent power source 24 is connected to the first terminal T1, and alsoconnected to the constant voltage power source 22. The constant voltagepower source 22 has a predetermined constant output voltage Vom of, forexample, 15 V, and the constant current power source 24 is operative tosupply, based on the constant output voltage Vom, a constant current tothe drive IC 20 via the terminal T1.

The drive IC 20 includes a charging switching element (SW) 26, adischarging switching element (SW) 30, a soft turnoff switching element(SW) 40, and a drive controller 44. As the charging switching element26, a P-channel MOSFET is used. In contrast, as each of the dischargingswitching element 28 and the soft-turnoff switching element 38, anN-channel MOSFET is used.

The constant current power source 24 is connected to the drain of thecharging switching element 26 via the first terminal T1, and the sourceof the charging switching element 26 is connected to the gate of theswitching element S*# via the second terminal T2. The constant currentpower source 24 and the charging switching element 26 according to thefirst embodiment serve as, for example, a charging unit.

The gate of the switching element S*# is connected to the emitterthereof via the discharging resistor 28, the third terminal T3, thedischarging switching element 30, and a common signal ground. Thedischarging switching element 28 has a predetermined resistance Rb.

The gate of the switching element S*# is connected to the emitterthereof via the soft turnoff resistor 38, the fourth terminal T4, thesoft turnoff switching element 40, and the common signal ground. Thesoft turnoff switching resistor 38 has a predetermined resistance Ra;the resistance Ra is set to be higher than the resistance Rb. The softturnoff resistor 38 and the soft turnoff switching element 40 serve as,for example, a soft turnoff unit.

Each switching element S*# has a sense terminal St for outputting aminute current, i.e. a sense current, associated with a current, i.e. acollector current Ic, flowing through the conductive path between theinput terminal and the output terminal thereof, i.e. between thecollector and the emitter thereof. For example, the magnitude of theminute current is 0.01%, i.e. one ten-thousandth of the collectorcurrent Ic.

The sense terminal St is connected to a first end of the sense resistor42, and a second end, opposing the first end, of the sense resistor 42is connected to the emitter of the switching element S*# via the commonsignal ground.

When the collector current Ic flows through the conductive path of eachswitching element S*#, the sense current correlating with the collectorcurrent Ic flows through the sense resistor 42, so that a voltage dropacross the sense resistor 42 occurs. Thus, it is possible to obtain thevoltage drop across the sense resistor 42 as a sense voltage Vse at thefirst end of the sense resistor 42 connected to the sense terminal St.The sense voltage Vse is an inter-terminal current parameter correlatingwith an electric state quantity of the magnitude of the collectorcurrent Ic flowing through the switching element S*#. That is, the levelof the sense voltage Vse serves as a function of, i.e. correlates with,the magnitude of the collector current Ic flowing through the switchingelement S*#.

In the first embodiment, the positive polarity of the sense voltage Vseaccording to the first embodiment is defined when the potential at thefirst end of the resistor 42 connected to the sense terminal St ishigher than the potential at the emitter of the switching element S*#.The potential at the emitter of the switching element S*# is set tozero, because the emitter of the switching element S*# is connected tothe common signal ground.

The gate of the switching element S*# is connected to the drivecontroller 44 via the fifth terminal T5, so that the gate voltage Vge ofthe switching element S*# is input to the drive controller 44 via thefifth terminal T5. The first end of the resistor 42 is connected to thedrive controller 44 via the sixth terminal T6, so that the sense voltageVse is input to the drive controller 44 via the sixth terminal T6. Thetemperature, referred to as Tmp, of the switching element S*# measuredby the temperature-sensitive sensor T*# is input to the drive controller44 via the seventh terminal T7.

In addition, the drive controller 44 is connected to the controller 14via the eighth terminal T8 and the interface 18, and is also connectedto the controller 14 via the ninth terminal T9 and the interface 18.

The drive controller 44 is operative to receive the drive signal g*#input thereto from the controller 14 via the interface 18 and theterminal T8. The drive signal g*# has one of a predetermined firstlogical level, i.e. a high level, defined as an on command, and apredetermined second logical level, i.e. a low level, defied as an offcommand.

The drive controller 44 is operative to alternately perform, based onthe drive signal g*#,

(1) The charging task for the switching element S*# by means ofswitching operations of the charging and discharging switching elements26 and 30

(2) The discharging task for the switching element S*# by means ofswitching operations of the charging and discharging switching elements26 and 30

The alternative charging and discharging tasks enable the switchingelement S*# to be driven.

Specifically, the drive controller 44 functionally includes a chargingunit 44 a and a discharging unit 44 b.

The charging unit 44 a of the drive controller 44 turns on, i.e. closes,the charging switching element 26 and turns off, i.e. opens, thedischarging switching element upon the drive signal g*# being changedfrom the off command to the on command. This enables a constant currentgenerated by the constant current power source 24 to be supplied to thegate of the switching element S*#, thus supplying electrical charge tothe gate of the switching element S*#. This enables the gate of theswitching element S*″ to be charged, so that the switching element S*#is turned on when the gate voltage Vge has reached the threshold voltageVth.

The discharging unit 44 b of the drive controller 44 turns on thedischarging switching element 30 and turns off the charging switchingelement 26 upon the drive signal g*# being changed from the on commandto the off command. This enables the gate of the switching element S*#to be discharged through the discharging switching element 30 via anormal gate discharging path defined between the gate and source of theswitching element S*# via the discharging resistor 28, the dischargingswitching element 30, and the common signal line. This changes theswitching element S*# from the on state to the off state when the gatevoltage Vge becomes lower than the threshold voltage Vth.

In addition, the drive controller 44 functionally includes anovercurrent determination unit 44 c that performs an overcurrentdetermination routine for a corresponding switching element S*# everypredetermined period during an overcurrent determination period. Theovercurrent determination period is defined between a first timing and asecond timing.

The first timing is defined at any timing within a period from thecharging start timing of the charging unit 44 a to the timing when thegate voltage Vge has reached a gate determination voltage Vjde describedlater assuming that an overcurrent flows through the switching elementS*#. For example, the first timing according to the first embodiment isset to the start timing of the charging task.

The second timing is defined at any timing within a period from apredetermined timing after the charging start timing to a timing beforethe timing when the gate voltage Vge has reached the gate determinationvoltage Vjde assuming that no overcurrent flows through the switchingelement S*#. For example, the second timing is set to an end timing of amiller period of the switching element S*# assuming that no overcurrentflows through the switching element S*#. Note that the Miller periodrepresents a period during which the gate voltage Vge remains at apredetermined constant voltage, i.e. a Miller voltage Vmil, while thedrain voltage rises or falls during the charging task or dischargingtask of the switching element S*#.

Upon starting the overcurrent determination routine for each switchingelement S*#, the drive controller 44, i.e. the overcurrent determinationunit 44 c, obtains a temperature parameter indicative of the temperatureTmp of the switching element S*#, such as the temperature Tmp of theswitching element S*# itself, from the temperature-sensitive sensor T*#in step S10. Note that the temperature Tmp according to the firstembodiment corresponds to, for example, a temperature measurement valueaccording to the first embodiment, and the operation in step S10corresponds to, for example, a temperature obtainer 44 c 1.

Next, the drive controller 44 sets a value of the gate determinationvoltage Vjde to be higher than the Miller voltage Vmil of the switchingelement S*# and to be lower than the output voltage Vom of the constantvoltage power source 22 in step S12. The reason why the gatedetermination voltage Vjde is set to be higher than the Miller voltageVmil is that it is to avoid an erroneous determination that anovercurrent is flowing through the switching element S*# under executionof the charging task although no overcurrent is flowing through theswitching element S*#.

The inventors of this application have considered the characteristics ofthe switching element S*#, and, as a result of this consideration, theinventors have found that, as illustrated in FIG. 4, the Miller voltageVlim has a negative correlation with the temperature Tmp, that is, thehigher the temperature Tmp is, the lower the Miller voltage Vlim is.

FIG. 4 schematically illustrates

(1) Correlation data G1 indicative of an example of the relationshipsbetween the temperature of the switching element S*# and the gatedetermination voltage Vjde, which is stored in, for example, the drivecontroller 44

(2) Correlation data G2 indicative of an example of the relationshipsbetween the temperature of the switching element S*# and the Millervoltage Vmil, which is stored in, for example, the drive controller 44

FIG. 4 schematically illustrates, by a solid line based on thecorrelation data G1, how the gate determination voltage Vjde is changeddepending on the temperature Tmp of the switching element S*#, and alsoillustrates, by a dashed line based on the correlation data G2, how theMiller voltage Vlim is changed depending on the temperature Tmp of theswitching element S*#.

In accordance with the inventor's finding, the gate determinationvoltage Vjde according to the first embodiment is set to be higher by apredetermined voltage difference ΔV than the Miller voltage Vlim withina predetermined operating temperature range of the switching elementS*#. That is, the gate determination voltage Vjde is set to becontinuously lower as the temperature Tmp of the switching element S*#becomes higher.

That is, FIG. 4 also schematically illustrates, by the solid line basedon the correlation data G1, how the gate determination voltage Vjde ischanged depending on the Miller voltage Vlim.

Note that the magnitude of the voltage difference ΔV is for example setto be slightly larger than the maximum amplitude of a noise voltage thatis assumed to be added to the gate voltage Vge. The voltage differenceΔV is set to be constant independently of the temperature Tmp of theswitching element S*# or set to vary depending on the temperature Tmp ofthe switching element S*#.

Specifically, FIG. 4 illustrates that the gate determination voltageVjde has a first voltage value Vjde1 when the temperature Tmp of theswitching element S*# is at a first temperature Tmp1. In addition, FIG.4 also illustrates that the gate determination voltage Vjde has a secondvoltage value Vjde2 lower than the first voltage value Vjde1 when thetemperature Tmp of the switching element S*# is at a second temperatureTmp2 which is higher than the first temperature Tmp1.

For example, in step S12, the drive controller 44 sets the gatedetermination voltage Vjde in accordance with the correlation data G1indicative of an example of the relationships between the temperature ofthe switching element S*# and the gate determination voltage Vjdeillustrated in FIG. 4 stored therein.

Specifically, in step S12, the drive controller 44 sets the gatedetermination voltage Vjde to the second voltage value Vde2 upon thetemperature Tmp of the switching element S*# is the second temperatureTmp2 higher than the first temperature Tmp1. The second voltage valueVde2 is lower than the first voltage value Vjde1 and higher than a valueVmil2 of the Miller voltage Vmil at the second temperature Tmp2. Notethat the operation in step S12 serves as, for example, a setter 44 c 2.

In step S14, the drive controller 44 obtains the gate voltage Vge of theswitching element S*#. The gate voltage Vge according to the firstembodiment corresponds to, for example, a terminal voltage at anopen-close control terminal of a switching element. The operation instep S14 corresponds to, for example, a voltage obtainer 44 c 3.

In step S16, the drive controller 44 determines whether the gate voltageVge is higher than the gate determination voltage Vjde.

Upon determining that the gate voltage Vge is higher than the gatedetermination voltage Vjde (YES in step S16), the drive controller 44obtains the sense voltage Vse in step S18. The sense voltage Vseaccording to the first embodiment corresponds to, for example, aninter-terminal current parameter, and the operation in step S18corresponds to, for example, a current parameter obtainer 44 c 4.

Following the operation in step S18, the drive controller 44 determineswhether the sense voltage Vse is higher than a predetermined sensedetermination voltage Vdec in step S20. The sense determination voltageVdec corresponds to an upper limit of the collector current Ic; thereliability of the switching element S*# can be maintained until thecollector current Ic is kept to be equal to or lower than its upperlimit. A value of the sense determination voltage Vdec is previouslydetermined for each switching element S*#. Note that the sensedetermination voltage Vdec corresponds to, for example a determinationcurrent parameter.

Upon determining that the sense voltage Vse is higher than thepredetermined sense determination voltage Vdec (YES in step S20), thatis, upon determining that the gate voltage Vge is higher than the gatedetermination voltage Vjde and the sense voltage Vse is higher than thesense determination voltage Vdec, the drive controller 44 determinesthat an overcurrent is flowing through the switching element S*# in stepS22.

Note that a condition of whether the gate voltage Vge is higher than thegate determination voltage Vjde corresponds to, for example, a firstcondition. In addition, a condition of whether the sense voltage Vse ishigher than the sense determination voltage Vdec corresponds to, forexample, a second condition. The operations in steps S16, S20, and S22correspond to, for example, a determiner 44 c 5.

The gate determination voltage Vjde serves as, for example, acurrent-related determination value.

Following the affirmative determination in step S22, the drivecontroller 44 performs an overcurrent protection task that

(1) Turns off or holds off both the charging switching element 26 anddischarging switching element 30

(2) Turns on the soft turnoff switching element 40 in step S24

This enables the electrical charge stored in the gate of the switchingelement S*# to be discharged through a gate discharging path definedfrom the gate of the switching element S*#, the soft turnoff resistor38, the soft turnoff switching element 40, the common signal ground, andthe emitter of the switching element S*#.

The overcurrent protection task discharges the electrical charge storedin the switching element S*# via the gate discharging path, thusforcibly turning off the switching element S*# slower than turnoff ofthe switching element S*# through the discharging switching element 28and discharging resistor 30. This is because the resistance Ra of thesoft turnoff resistor 38 is higher than the resistance Rb of thedischarging resistor 30.

If the switching speed of the switching element S*# from the on state tothe off state were high while an overcurrent is flowing through theswitching element S*# as the corrector current Ic, there might be anexcessively high surge voltage produced due to the high-speed shutdownof the switching element S*#. Thus, in this situation, the overcurrentprotection task forcibly changes the resistance of the gate dischargingpath of the gate of the switching element S*# via the soft turnoffresistor 38 to be higher than that of the normal gate discharging pathof the gate of the switching element S*# via the discharging resistor 28in normal situations. This protects the switching element S*# against asurge produced due to the turn-off of the switching element S*#.

When executing the overcurrent protection task, the drive controller 44executes a task to output a fail-safe signal FL to the controller 14 viathe ninth terminal T9 and the interface 18 in step S26. The fail-safesignal FL represents an abnormal state in the operation of the switchingelement S*#. In response to the fail-safe signal FL, the controller 14shuts down the inverter 11.

Otherwise, upon determining that the gate voltage Vge is equal to orlower than the gate determination voltage Vjde (NO in step S14) or thatthe sense voltage Vse is equal to or lower than the predetermined sensedetermination voltage Vdec (NO in step S20), the drive controller 44determines that no overcurrent is flowing through the switching elementS*# in step S28, terminating the overcurrent determination routine.

Note that the drive controller 44 can be comprised of, for example, aprocessor and a processor-readable memory, such as a processor-readablenonvolatile memory storing program instructions. That is, the processorcan run the program instructions to thereby execute the operations insteps S1 to S14, in other words, to thereby implement at least themodules 44 c 1 to 44 c 5. As another example, the drive controller 44can be comprised of hardwired logic circuits that can implement theoperations in step S1 to S14, that is, that can respectively serve as atleast the modules 44 c 1 to 44 c 5. As a further example, the drivecontroller 44 can be comprised of hardwired/programmed hybrid logiccircuits that can implement the operations in step S1 to S14, that is,that can respectively serve as at least the modules 44 c 1 to 44 c 5.

Next, the following describes how an example of the overcurrentprotection task is performed if no overcurrent is flowing through atarget switching element S*# in a normal condition with reference toFIGS. 5A to 5D first. In addition, the following describes how theexample of the overcurrent protection task is performed if anovercurrent is flowing through the target switching element S*# due to,for example, the occurrence an upper-lower arm short-circuit in thetarget switching element in an abnormal condition with reference toFIGS. 6A to 6D next.

FIGS. 5A to 5E are a joint timing chart schematically illustrating theexample of the overcurrent protection task in the normal condition.

Specifically, FIG. 5A schematically illustrates an example of how thegate voltage Vge of the target switching element S*# is changed overtime, and FIG. 5B schematically illustrates an example of how thecharging switching element 26 is driven over time. FIG. 5C schematicallyillustrates an example of how the soft-turnoff switching element 40 isdriven over time, and FIG. 5D schematically illustrates an example ofhow the sense voltage Vse is changed over time.

In particular, in FIG. 5A, a dashed graph F1 represents how the gatevoltage Vge is changed over time upon the temperature Tmp of the targetswitching element S*# being the first temperature Tmp1. In contrast, asolid graph F2 represents how the gate voltage Vge is changed over timeupon the temperature Tmp of the target switching element S*# being thesecond temperature Tmp2.

The following describes the example of the overcurrent protection taskin the normal condition using the solid graph F2.

Referring to FIG. 5, when the charging task is started at time t1, thedischarging switching element 30 is turned off, and the chargingswitching element 26 is turned on. This causes a constant currentgenerated by the constant current power source 24 to be supplied to thegate of the switching element S*#, so that the gate voltage Vge startsincreasing.

Thereafter, when the gate voltage Vge reaches the threshold voltage Vthat time t2, the sense voltage Vse starts increasing.

After the start of an increase of the sense voltage Vse, the gatevoltage Vge reaches the Miller voltage Vmil at time t3. At that time,the gate voltage Vge remains at the Miller voltage Vmil until the Millerperiod has elapsed since the time t3, because no overcurrent is flowingthrough the target switching element S*#. For this reason, while theMiller period has elapsed since the time t3, the gate voltage Vge issubstantially maintained at the Miller voltage Vmil, so that the gatevoltage Vge is kept to be equal to or lower than the second voltagevalue Vjde2. This prevents the sense voltage Vse from exceeding thesense determination voltage Vdec. After the time t4, the gate voltageVge has reached the output voltage Vom of the constant voltage powersource 22.

Note that, although a Miller voltage arrival timing t3 a of the graph F1is different from the Miller voltage arrival timing t3, and a Millarperiod end timing t4 a of the graph F1 is different from the Millerperiod end timing t4, the whole behavior of the graph F1 issubstantially similar to the whole behavior of the graph F2. So, theredundant descriptions of the graph F1 are omitted.

Next, the following describes the example of the overcurrent protectiontask in the abnormal condition.

FIGS. 6A to 6E respectively correspond to FIGS. 5A to 5E.

In particular, in FIG. 6A, a dashed graph F1A represents how the gatevoltage Vge is changed over time upon the temperature Tmp of the targetswitching element S*# being the first temperature Tmp1. In contrast, asolid graph F2A represents how the gate voltage Vge is changed over timeupon the temperature Tmp of the target switching element S*# being thesecond temperature Tmp2.

In addition, in FIG. 6B, a dashed graph F1B represents how the chargingswitching element 26 is driven over time upon the temperature Tmp of thetarget switching element S*# being the first temperature Tmp1. Incontrast, a solid graph F2B represents how the charging switchingelement 26 is driven over time upon the temperature Tmp of the targetswitching element S*# being the second temperature Tmp2.

Referring to FIG. 6, when the charging task is started at time t11, thedischarging switching element 30 is turned off, and the chargingswitching element 26 is turned on. This causes a constant currentgenerated by the constant current power source 24 to be supplied to thegate of the target switching element S*#, so that the gate voltage Vgestarts increasing.

Thereafter, when the gate voltage Vge reaches the threshold voltage Vthat time t12, the sense voltage Vse starts increasing.

After the start of an increase of the sense voltage Vse, because thereis no Miller period in the gate voltage Vge while an overcurrent isflowing through the target switching element S*#, the gate voltage Vgecontinuously increases after the time t11 without remaining at theMiller voltage Vmil.

This results in

(1) The gate voltage Vge reaching the second voltage value Vjde2 at timet13 upon the temperature Tmp of the switching element S*# being thesecond temperature Tmp2 (see the graph F1A)

(2) The gate voltage Vge reaching the first voltage value Vjde1 at timet15 upon the temperature Tmp of the switching element S*# being thefirst temperature Tmp1 (see the graph F2A)

On the other hand, the sense voltage Vse, which has increased since thetime t12, continuously increases to reach the sense determinationvoltage Vdec at time t14 while the overcurrent is flowing through thetarget switching element S*#.

The drive controller 44 stores beforehand a border voltage Vbor. Theborder voltage Vbor is a voltage value at which the gate voltage Vgearrives when the sense voltage Vse reaches the sense determinationvoltage Vdec at the time t14 assuming that an overcurrent is flowingthrough the switching element S*#.

In other words, the border voltage Vbor is a value of the gate voltageVge when the collector current Ic defined based on the sense voltage Vsereaches a determination current defined based on the sense determinationvoltage Vdec at the time t14 when it is assumed that an overcurrent isflowing through the corresponding switching element S*#.

In particular, the first voltage value Vjde1 according to the firstembodiment is set to be higher than the border voltage Vbor, and thesecond voltage value Vjde2 according to the first embodiment is set tobe lower than the border voltage Vbor. Note that the time t14 accordingto the first embodiment corresponds to, for example, an arrival timing.

This setting of the first and second determination voltages Vjde1 andVjde2 results in

(1) The time t13 at which the gate voltage Vge reaches the seconddetermination voltage Vjde2 being earlier than the time t14 at which thegate voltage Vge reaches the border voltage Vbor

(2) The time t15 at which the gate voltage Vge reaches the firstdetermination voltage Vjde1 being later than the time t14 at which thegate voltage Vge reaches the border voltage Vbor

This results in the sense voltage Vse reaching the sense determinationvoltage Vdec at the time t14 upon the temperature Tmp of the switchingelement S*# being the first temperature Tmp1. Thereafter, the sensevoltage Vse reaches the first voltage value Vjde1 at the time t15 uponthe temperature Tmp of the switching element S*# being the firsttemperature Tmp1. This therefore enables the drive controller 44 todetermine that an overcurrent is flowing through the target switchingelement S*# at the time t15 upon the temperature Tmp of the switchingelement S*# being the first temperature Tmp1 (see step S22).

In contrast, upon the temperature Tmp of the switching element S*# beingthe second temperature Tmp2, the gate voltage Vge reaches the seconddetermination voltage Vjde2 at the time t13, and thereafter, the sensevoltage Vdec reaches the sense determination voltage Vdec at the timet14. That is, upon the temperature Tmp of the switching element S*#being the second temperature Tmp2, the drive controller 44 determinesthat an overcurrent is flowing through the target switching element S*#at the time t14 (see step S22). That is, the drive controller 44determines that

(1) An overcurrent is flowing through the target switching element S*#upon the temperature Tmp of the switching element S*# being the firsttemperature Tmp1 at a first timing

(2) An overcurrent is flowing through the target switching element S*#upon the temperature Tmp of the switching element S*# being the secondtemperature Tmp2 at a second timing earlier than the first timing.

As described in detail above, the drive controller 44 for each switchingelement S*# according to the first embodiment is configured to performthe overcurrent determination routine for the corresponding switchingelement S*# that sets, upon the temperature Tmp of the correspondingswitching element S*# being the second temperature Tmp2 higher than thefirst temperature Tmp1, the gate determination voltage Vjde to thesecond voltage value Vjde2 that is lower than the first voltage valueVjde1; the first voltage value Vjde1 is previously set for the firsttemperature Tmp1 of the corresponding switching element S*#. In theovercurrent determination routine, the lower the gate determinationvoltage Vjde is, the earlier whether an overcurrent is flowing throughthe corresponding switching element S*# is determined.

This configuration therefore enables whether an overcurrent is flowingthrough each switching element S*# to be determined earlier as comparedwith the case where the second voltage value Vjde2 is set to be the sameas the first voltage value Vjde1.

This earlier determination of an overcurrent flowing results in areduction of an integrated quantity of the collector current Ic that isflowing through each switching element S*# since the start of thecharging task until it is determined that an overcurrent is flowingthrough the switching element S*#. This prevents a large amount ofcollector current Ic from flowing between the collector and emitter ofeach switching element S*#, thus preventing the reliability of eachswitching element S*# from deteriorating.

This earlier determination of an overcurrent flowing also prevents eachhigh-side switching element S*p and the corresponding low-side switchingelement S*n from being simultaneously turned on, thus preventing thereliability of the motor-generator 10 from deteriorating.

The drive controller 44 for each switching element S*# according to thefirst embodiment is configured to determine whether an overcurrent isflowing through each switching element S*# using both the gate voltageVge and the sense voltage Vse of the corresponding switching elementS*#. This improves the reliability of the overcurrent detection accuracyas compared with the configuration that determines whether anovercurrent is flowing through each switching element S*# using merelythe gate voltage Vge of the corresponding switching element S*#.

For example, even if a larger amplitude noise voltage than a noisevoltage that is assumed to be added to the gate voltage Vge is appliedto the gate voltage Vge of each switching element S*# so that the gatevoltage Vge temporarily has exceeded the gate determination voltage Vjdewhile no overcurrent is flowing through the corresponding switchingelement S*#, the drive controller 44 according to the first embodimentprevents an erroneous determination that an overcurrent is flowingthrough the corresponding switching element S*#.

In particular, the second voltage value Vjde2 according to the firstembodiment is set to be lower than the border voltage Vbor at which thegate voltage Vge of each switching element S*# is assumed to arrive whenthe sense voltage Vse reaches the sense determination voltage Vse whilean overcurrent is flowing through the corresponding switching elementS*#. This configuration enables whether an overcurrent is flowingthrough each switching element S*# is determined earlier than the casewhere the second voltage value Vjde2 is set to be higher than the bordervoltage Vbor.

The soft turnoff resistor 38 according to the first embodiment has theresistance Ra set to be higher than the resistance Rb of the dischargingresistor 28. This enables the discharging rate of charge stored in thegate of each switching element S*# via the soft-turnoff discharge pathincluding the soft turnoff resistor 38 to be lower than the dischargingrate via the normal discharging path including the discharging resistor28.

If the switching speed of the switching element S*# from the on state tothe off state were high while an overcurrent is flowing through theswitching element S*# as the corrector current Ic, there might be anexcessively high surge voltage produced due to the high-speed shutdownof the switching element S*#. Thus, in this situation, the overcurrentprotection task forcibly changes the resistance of the discharging pathof the gate of each switching element S*# via the soft turnoff resistor38 to be higher than that of the normal discharging path of the gate ofthe corresponding switching element S*# via the discharging resistor 28in normal situations. This protects each switching element S*# against asurge produced due to the turn-off of the corresponding switchingelement S*#.

Second Embodiment

The following describes a drive unit DUA for each switching element S*#according to the second embodiment of the present disclosure withreference to FIGS. 7 and 8.

The structure and/or functions of the drive unit DUA according to thesecond embodiment are mainly identical to those of the drive unit DUaccording to the first embodiment except for the following points. So,the following describes mainly the different points.

From the drive unit DUA for each switching element S*# according to thesecond embodiment, the sense terminal, the sense resistor 42, and thesixth terminal T6 of the drive IC 20 has been eliminated as comparedwith the drive unit DU according to the first embodiment.

That is, the drive unit DUA according to the second embodiment isconfigured to deter mine whether an overflow current is flowing througheach switching element S*# using the gate voltage Vge.

The following describes an overcurrent determination routine accordingto the second embodiment with reference to FIG. 8. A drive controller44A executes the overcurrent determination routine for each switchingelement S*# every predetermined period during the overcurrentdetermination period.

Note that, in the overcurrent determination routines illustrated inrespective FIGS. 3 and 8, like operations between the overcurrentdetermination routines illustrated in respective FIGS. 3 and 8, to whichlike step numbers are assigned, are omitted or simplified to avoidredundant description.

From the overcurrent determination routine illustrated in FIG. 8, theoperations in steps S18 and S20 have been eliminated as compared withthe overcurrent determination routine illustrated in FIG. 3.

Specifically, upon determining that the gate voltage Vge is higher thanthe gate determination voltage Vjde (YES in step S16), the drivecontroller 44 determines that an overcurrent is flowing through theswitching element S*# in step S22.

Specifically, the drive controller 44A for each switching element S*#according to the second embodiment is configured to simply determinethat an overcurrent is flowing through each switching element S*# upondetermining that the gate voltage Vge has exceeded the gatedetermination voltage Vjde.

This configuration therefore enables, upon the gate determinationvoltage Vjde being set to be lower than the border voltage Vbor, whetheran overcurrent is flowing through each switching element S*# to bedetermined earlier as compared with the case where the drive controller44 performs the determination of whether an overcurrent is flowingthrough each switching element S*# using both the gate voltage Vge andthe sense voltage Vse.

In particular, the drive controller 44 for each switching element S*#according to the first embodiment is configured to determine whether anovercurrent is flowing through each switching element S*# using merelythe gate voltage Vge of the corresponding switching element S*#. Thiseliminates the sense terminal St, the sense resistor 42, and the sixthterminal T6, which are needed in the first embodiment. This results inthe drive unit DUA having a simpler structure, resulting therefore in asmaller circuit board on which each drive unit DUA is mounted.

Third Embodiment

The following describes a drive unit DUB for each switching element S*#according to the third embodiment of the present disclosure withreference to FIGS. 9 and 10.

The structure and/or functions of the drive unit DUB according to thethird embodiment are mainly identical to those of the drive unit DUaccording to the first embodiment except for the following points. So,the following describes mainly the different points.

Referring to FIG. 9, each switching element S*# according to the thirdembodiment is comprised of a first switching element S*#1 and a secondswitching element S*#2 parallely connected to each other. Specifically,each of first switching element S*#1 and a second switching element S*#2is comprised of an IGBT, and the output terminals, i.e. emitters, of thefirst and second switching elements S*#1 and S*#2 are connected to eachother, and the input terminals, i.e. collectors, of the first and secondswitching elements S*#1 and S*#2 are also connected to each other.

Referring to FIG. 9, the drive unit DUB for each switching element S*#includes a first gate resistor 46 and a second gate resistor 48.

The gate of the first switching element S*#1 is connected to the secondterminal T2 of a drive controller 44B via the first gate resistor 46.This results in each of the discharging resistor 28 and the soft turnoffresistor 38 being connected to the gate of the first switching elementS*#1 via the first gate resistor 46.

Additionally, the gate of the second switching element S*#2 is connectedto the second terminal T2 via the second gate resistor 48. This resultsin each of the discharging resistor 28 and the soft turnoff resistor 38being also connected to the gate of the second switching element S*#2via the second gate resistor 48.

That is, the gates of the first and second switching elements S*#1 andS*#2 are commonly connected to the second terminal T2 via the respectivefirst and second gate resistors 46 and 48. Note that the second terminalT2 serves as, for example, a charging terminal.

This connection configuration enables a gate voltage Vge1 at the gate ofthe first switching element S*#1 and a gate voltage Vge2 at the gate ofthe second switching element S*#2 to be estimated based on the voltageat the second terminal T2. The second terminal T2 according to the thirdembodiment is connected to the drive controller 44B via the fifthterminal T5, so that the voltage at the second terminal T2 is input tothe drive controller 44B. This enables the drive controller 44B tosimultaneously obtain both the first and second gate voltages Vge1 andVge2 based on the voltage at the second terminal T2 input thereto.

The inverter 11 according to the third embodiment includes firstflywheel diodes D*#1 electrically connected in antiparallel to therespective first switching elements S*#1, and also includes secondflywheel diodes D*#2 electrically connected in antiparallel to therespective second switching elements S*#2.

The inverter 11 also includes first temperature-sensitive sensors T*#1located to be close to the respective first switching elements S*#1, andincludes second temperature-sensitive sensors T*#2 located to be closeto the respective second switching elements S*#2. Specifically, eachfirst semiconductor switch Ze1 is comprised of the corresponding firstswitching element S*#1, the corresponding first flywheel diode D*#1, andthe corresponding first temperature-sensitive sensor T*#1 packagedtherein. In addition, each second semiconductor switch Ze2 is comprisedof the corresponding second switching element S*#2, the correspondingsecond flywheel diode D*#2, and the corresponding secondtemperature-sensitive sensor T*#2 packaged therein.

The drive unit DUB includes a drive IC 20B that additionally has tenthand eleventh terminals T10 and T11, and includes a drive controller 44B.

The sense resistor 42 according to the first embodiment serves as afirst sense resistor 42 provided for each first switching element S*#1,so that the first end of the first sense resistor 42 is connected to thesense terminal St of the corresponding first switching element S*#1, andthe second end of the first sense resistor 42 is connected to the commonsignal ground. The first end of the first sense resistor 42 is connectedto the sixth terminal T6 of the drive controller 44B.

This enables the drive controller 44B to obtain the voltage drop acrossthe first sense resistor 42 as a first sense voltage value Vse1 at thefirst end of the first sense resistor 42 connected to the sense terminalSt.

Each drive unit DUB additionally includes a second sense resistor 50provided for each second switching element S*#2, so that the first endof the second sense resistor 50 is connected to the sense terminal St ofthe corresponding second switching element S*#2, and the second end ofthe second sense resistor 50 is connected to the common signal ground.The first end of the second sense resistor 50 is connected to the tenthterminal T10 of the drive controller 44B.

This enables the drive controller 44B to obtain the voltage drop acrossthe second sense resistor 52 as a second sense voltage value Vse2 at thefirst end of the second sense resistor 50 connected to the senseterminal St.

The temperature Tmp of the first switching element S*#1 measured by thefirst temperature-sensitive sensor T*#1 is input to the drive controller44B via the seventh terminal T7. Similarly, the temperature, referred toas Xmp, of the second switching element S*#2 measured by the secondtemperature-sensitive sensor T*#2 is input to the drive controller 44Bvia the seventh terminal T11.

Note that, for the sake of simply illustration of the drive unit DUB,the switching elements 26, 30, and 40 are not illustrated, but the driveunit DUB includes the switching elements 26, 30, and 40, which issimilar to the drive unit DU of the first embodiment.

The following describes an overcurrent determination routine accordingto the third embodiment with reference to FIG. 10. The drive controller44B executes the overcurrent determination routine for each pair of thefirst and second switching elements S*#1 and S*#2 every predeterminedperiod during the overcurrent determination period.

Note that, in the overcurrent determination routines illustrated inrespective FIGS. 3 and 10, like operations between the overcurrentdetermination routines illustrated in respective FIGS. 3 and 10, towhich like step numbers are assigned, are omitted or simplified to avoidredundant description.

Upon starting the overcurrent determination routine for each pair of thefirst and second switching elements S*#1 and S*#2, the drive controller44B, i.e. the overcurrent determination unit 44 c, obtains a firsttemperature parameter indicative of the temperature Tmp of the switchingelement S*#, such as the temperature Tmp of the first switching elementS*#1 itself, from the first temperature-sensitive sensor T*#1 in stepS40.

In step S40, the drive controller 44B also obtains a second temperatureparameter indicative of the temperature Xmp of the second switchingelement S*#2, such as the temperature Xmp of the second switchingelement S*#2 itself, from the second temperature-sensitive sensor T*#2in step S40. Note that the temperature Tmp according to the thirdembodiment corresponds to, for example, a first temperature measurementvalue according to the third embodiment, and the temperature Xmpaccording to the third embodiment corresponds to, for example, a secondtemperature measurement value according to the third embodiment. Theoperation in step S40 corresponds to, for example, a temperatureobtainer 44 c 11.

Next, the drive controller 44B sets a value of a gate determinationvoltage Vjde for the first switching element S*#1, and sets a value of agate determination voltage Yjde for the second switching element S*#2 instep S42.

For example, in step S42, the drive controller 44B sets the gatedetermination voltages Vjde and Yjde in accordance with the correlationdata G1A and the correlation data G2A described above and stored in FIG.11 stored therein. Note that the operation in step S42 serves as, forexample, a setter 44 c 12.

In FIG. 11, the correlation data G1A shows that a variable for each ofthe gate determination voltages Vjde and Yjde has a negative correlationwith the temperature Tmp, Xmp of the corresponding switching element.

The correlation data G2A shows that the Miller voltage Vmil for each ofthe first and second switching elements S*#1 and S*#2 has a negativecorrelation with the temperature Tmp, Xmp of the corresponding switchingelement.

That is, the higher the temperature Tmp, Xmp for each of the first andsecond switching elements S*#1 and S*#2 is, the lower the gatedetermination voltage Vjde, Yjde for the corresponding one of the firstand second switching elements S*#1 and S*#2 is. Similarly, the higherthe temperature Tmp for each of the first and second switching elementsS*#1 and S*#2 is, the lower the Miller voltage for the corresponding oneof the first and second switching elements S*#1 and S*#2 is.

For this reason, when the first temperature Tmp1 is set to be lower thanthe second temperature Tmp2, the first voltage value Vjde1 correspondingto the temperature Tmp of each of the first and second switchingelements S*#1 and S*#2 being the first temperature Tmp1 is higher thanthe second voltage value Vjde2 corresponding to the temperature Tmp ofeach of the first and second switching elements S*#1 and S*#2 being thesecond temperature Tmp2.

Specifically, in step S42, the drive controller 44B compares thetemperature Tmp of the first switching element S*#1 with the temperatureXmp of the second switching element S*#2, and selects the lowertemperature in the temperatures Tmp and Xmp. Then, the drive controller44B refers to the correlation data G1A to thereby select a value of thevariable for each of the gate determination voltages Vjde and Yjde atthe selected temperature as the corresponding one of the gatedetermination voltages Vjde and Yjde in step S42.

For example, in step S42, the drive controller 44B sets each of the gatedetermination voltages Vjde and Yjde to the first voltage value Vjde1corresponding to the first temperature Tmp1 assuming that thetemperature Tmp of the first switching element S*#1 being the firsttemperature Tmp1, and the temperature Xmp of the second switchingelement S*#2 being the second temperature Tmp2.

In step S44, the drive controller 44B obtains the voltage at the secondterminal T2 to thereby obtain the first gate voltage Vge1 of the firstswitching element S*#1 and the second gate voltage Vge2 of the secondswitching element S*#2. The operation in step S44 corresponds to, forexample, a voltage obtainer 44 c 13.

In step S46, the drive controller 44B determines whether the first gatevoltage Vge1 is higher than the gate determination voltage Vjde.

Upon determining that the first gate voltage Vge1 is higher than thegate determination voltage Vjde (YES in step S46), the drive controller44B obtains the first sense voltage value Vse1 in step S48. Then, thedrive controller 44B determines whether the first sense voltage valueVse1 is higher than the determination voltage Vdec in step S50. Theoperation in step S48 corresponds to, for example, a current parameterobtainer 44 c 14.

Upon determining that the first sense voltage value Vse1 is higher thanthe sense determination voltage Vdec (YES in step S50), that is, upondetermining that the first gate voltage Vge1 is higher than the gatedetermination voltage Vjde and the first sense voltage value Vse1 ishigher than the sense determination voltage Vdec, the drive controller44B determines that an overcurrent is flowing through the firstswitching element S*#1 in step S22.

Otherwise, upon determining that the first gate voltage Vge1 is equal toor lower than the gate determination voltage Vjde (NO in step S46) orthat the first sense voltage value Vse1 is equal to or lower than thesense determination voltage Vdec (NO in step S50), the drive controller44B determines whether the second gate voltage Vge2 is higher than thegate determination voltage Yjde in step S52.

Upon determining that the second gate voltage Vge2 is higher than thegate determination voltage Yjde (YES in step S52), the drive controller44B obtains the second sense voltage value Vse2 in step S54. Then, thedrive controller 44B determines whether the second sense voltage valueVse2 is higher than the determination voltage Vdec in step S56.

Upon determining that the second sense voltage value Vse2 is higher thanthe sense determination voltage Vdec (YES in step S56), that is, upondetermining that the second gate voltage Vge2 is higher than the gatedetermination voltage Yjde and the second sense voltage value Vse2 ishigher than the sense determination voltage Vdec, the drive controller44B determines that an overcurrent is flowing through the secondswitching element S*#2 in step S22.

Otherwise, upon determining that the second gate voltage Vge2 is equalto or lower than the gate determination voltage Yjde (NO in step S52) orthat the second sense voltage Vse is equal to or lower than the sensedetermination voltage Vdec (NO in step S56), the drive controller 44Bdetermines that no overcurrent is flowing through the switching elementS*#, terminating the overcurrent determination routine.

Note that the drive controller 44B can be comprised of, for example, aprocessor and a processor-readable memory, such as a processor-readablenonvolatile memory, for example, a non-transitory memory, storingprogram instructions. That is, the processor can run the programinstructions to thereby execute the operations in steps S40 to S56 andS22 to S26, in other words, to thereby implement at least the modules 44c 11 to 44 c 14 and 44 c 5. As another example, the drive controller 44Bcan be comprised of hardwired logic circuits that can implement theoperations in steps S40 to S56 and S22 to S26, that is, that canrespectively serve as at least the modules 44 c 11 to 44 c 14 and 44 c5. As a further example, the drive controller 44 can be comprised ofhardwired/programmed hybrid logic circuits that can implement theoperations in steps S40 to S56 and S22 to S26S, that is, that canrespectively serve as at least the modules 44 c 11 to 44 c 14 and 44 c5.

As described above, the drive controller 44B for each of the first andsecond switching elements S*#1 and S*#2 according to the thirdembodiment is configured to set each of the gate determination voltageVjde for the first switching element S*#1 and the gate determinationvoltage Yjde for the second switching element S*#2 to a same voltage.This therefore results in lower processing load of the drive controller44B during execution of the overcurrent determination routine ascompared with the case where the drive controller 44B sets the gatedetermination voltage Vjde for the first switching element S*#1 and thegate determination voltage Yjde to respectively different voltages.

In particular, the drive controller 44B according to the thirdembodiment is configured to set each of the gate determination voltageVjde for the first switching element S*#1 and the gate determinationvoltage Yjde for the second switching element S*#2 in accordance withthe lower temperature in the temperature Tmp of the first switchingelement S*#1 and the temperature Xmp of the second switching elementS*#2.

For example, as illustrated in FIG. 11, it is assumed that

(1) The temperature Tmp of the first switching element S*#1 is the firsttemperature Tmp1

(2) The temperature Xmp of the second switching element S*#2 is thesecond temperature Tmp2

(3) Each of the gate determination voltages Vjde and Yjde is set to thesecond voltage value Vjde2 corresponding to the higher temperature inthe temperature Tmp of the first switching element S*#1 and thetemperature Xmp of the second switching element S*#2

In this assumption, the second voltage value Vjde2 set for the gatedetermination voltage Vjde of the first switching element S*#1 would belower than the Miller voltage Vmil1 at the temperature Tmp of the firstswitching element S*#1. This would cause the driver controller 44B toerroneously determine that an overcurrent is flowing through the firstswitching element S*#1.

In contrast, the drive controller 44B according to the third embodimentis configured to set each of the gate determination voltage Vjde for thefirst switching element S*#1 and the gate determination voltage Yjde forthe second switching element S*#2 in accordance with the lowertemperature in the temperature Tmp of the first switching element S*#1and the temperature Xmp of the second switching element S*#2. Thisresults in reduction of the possibility of such an erroneousdetermination.

The drive unit DUB according to the third embodiment is configured suchthat the gates of the first and second switching elements S*#1 and S*#are commonly connected to the second terminal T2 via the respectivefirst and second gate resistors 46 and 48.

This connection configuration enables the drive controller 44B tosimultaneously obtain both the first and second gate voltages Vge1 andVge2 based on the voltage at the second terminal T2 input thereto. Thisresults in the drive controller 44B having one terminal, i.e. the fifthterminal T5, for obtaining both the first and second gate voltages Vge1and Vge2. This therefore results in a reduction of the number ofterminals needed for the drive controller 44B as compared with theconfiguration that a drive controller has two terminals for respectivelyobtaining the first and second gate voltages Vge1 and Vge2 in additionto the above benefits obtained in the first embodiment.

Fourth Embodiment

The following describes a drive unit DUC for each switching element S*#according to the fourth embodiment of the present disclosure withreference to FIG. 12.

The structure and/or functions of the drive unit DUC according to thefourth embodiment are mainly identical to those of the drive unit DUBaccording to the third embodiment except for the following points. So,the following describes mainly the different points.

Referring to FIG. 12, the fifth terminal T5 of a drive IC 20 c of thedrive unit DUC is connected to a signal line between the first gateresistor 46 and the gate of the first switching element S*#1. Inaddition, the drive IC 20 c of the drive unit DUC has a twelfth terminalT12 connected to a signal line between the second gate resistor 48 andthe gate of the second switching element S*#2.

This enables a drive controller 44C of the drive unit DUC toindividually obtain the first and second gate voltages Vge1 and Vge2through the respective fifth and twelfth terminals T5 and T12.

The drive controller 44C is configured to execute an overcurrentdetermination routine according to the fourth embodiment, which isslightly different from the overcurrent determination routine accordingto the third embodiment in the following points.

Specifically, in step S42, the drive controller 44C individually setsthe gate determination voltages Vjde and Yjde in accordance with thecorrelation data G1A described above and stored in FIG. 11 storedtherein.

Specifically, in step S42, the drive controller 44C sets, in accordancewith the correlation data G1A, the gate determination voltage Vjde forthe first switching element S*#1 to a voltage value corresponding to thetemperature Tmp of the first switching element S*#1 in accordance withthe correlation data G1A. In addition, in step S42, the drive controller44C sets, in accordance with the correlation data G1A, the gatedetermination voltage Yjde for the second switching element S*#2 to avoltage value corresponding to the temperature Xmp of the secondswitching element S*#2 in accordance with the correlation data G1A.

As described above, the drive unit DUC according to the fourthembodiment is configured to individually and directly obtain the firstand second gate voltages Vge1 and Vge2 through the respective fifth andtwelfth terminals T5 and T12 with little influence from the gateresistors 46 and 48.

This configuration therefore enables the first and second gate voltagesVge1 and Vge2 to be obtained with higher accuracy.

The drive unit DUC is configured to individually set the gatedetermination voltages Vjde and Yjde in accordance with the correlationdata G1A. This configuration enables the gate determination voltagesVjde and Yjde to be set based on the temperatures of the respectivefirst and second switching elements S*#1 and S*#2.

Modifications

The present disclosure is not limited to the above first to fourthembodiments, and each of the first to fourth embodiments of the presentdisclosure can be modified as described hereinafter.

Each of the first to fourth embodiments is configured to address asituation where there is no Miller period in the change trajectory ofthe gate voltage Vge of a switching element S*# from the off state tothe on state due to the flow of an overcurrent through the switchingelement S*#. The present disclosure is however not limited to thisconfiguration.

Specifically, each of the first to fourth embodiments can be configuredto address a situation where there is a shorter Miller period in thechange trajectory of the gate voltage Vge of a switching element S*#from the off state to the on state due to the magnitude of anovercurrent flowing through the switching element S*#. Specifically,while an overcurrent is flowing through a switching element S*#, thegate voltage Vge of the switching element S*# becomes higher than thegate determination voltage Vjde and the sense voltage Vse becomes higherthan the sense determination voltage Vse during a predeterminedovercurrent determination period. For this reason, each of the first tofourth embodiments makes it possible to determine that there is anovercurrent flowing through each switch S*# independently of themagnitude of an overcurrent flowing through the switching element S*#.

Each of the drive controllers 44 and 44A to 44C is configured to obtainthe temperature Tmp of each switching element S*# itself, but thepresent disclosure is not limited thereto.

Specifically, each of the drive controllers 44 and 44A to 44C can beconfigured to

(1) Obtain, as the temperature parameter of each switching element S*#,a physical value, such as a voltage, correlating with the temperatureTmp of each switching element S*# or

(2) Obtain, as each of the first and second temperature parameters ofthe respective first and second switching elements S*#1 and S*#2, aphysical value, such as a voltage, correlating with the correspondingone of the temperatures Tmp and Xmp of the corresponding one of thefirst and second switching elements S*#1 and S*#2

For example, a thermoelectric conversion element can be provided foreach switching element S*#, and the thermoelectric conversion elementcan be configured to output a voltage correlating with the temperatureof the corresponding switching element S*#. Then, each of the drivecontrollers 44 and 44A to 44C can be configured to obtain the voltagecorrelating with the temperature of each corresponding switching elementS*#, thus obtaining the temperature of the corresponding switchingelement S*#.

Each of the drive controllers 44 and 44A to 44C is configured to obtainthe sense voltage Vs of each switching element S*# as a currentparameter correlating with an inter-terminal current, such as thecollector current Ic, flowing through between the pair of main terminalsof the corresponding switching element S*#, but the present disclosureis not limited thereto.

Specifically, each of the drive controllers 44 and 44A to 44C can beconfigured to obtain, as the current parameter, a current flowingbetween the sense terminal and the emitter of each switching elementS*#, which correlates with the inter-terminal current, measured by, forexample, a current sensor included in the corresponding drive unit. Asanother example, each of the drive controllers 44 and 44A to 44C can beconfigured to obtain, as the current parameter, a collector-emittervoltage, which correlates with the collector current Ic, measured by,for example, a voltage sensor included in the corresponding drive unit.

Each of the first and second embodiments is configured to continuouslyreduce the gate determination voltage of each switching element S*# downto the second voltage value Vjde2, which is lower than the first voltagevalue Vjde1, with an increase of the measured temperature Tmp of thecorresponding switching element S*# when the measured temperature Tmp ofthe corresponding switching element S*# is the second temperature Tmp2higher than the first temperature Tmp1. The present disclosure ishowever not limited to the configuration. For example, the presentdisclosure can be configured to set the gate determination voltage Vjdeof each switching element S*# to be stepwisely lower as the temperatureTmp of the corresponding switching element S*# becomes higher. Note thatthe present disclosure can be configured to set the gate determinationvoltage Vjde of each switching element S*# to be stepwisely lower in atleast two steps as the temperature Tmp of the corresponding switchingelement S*# becomes higher.

Each of the drive units DUB and DUC according to the third and fourthembodiments can include each switching element S*# comprised of twoswitching elements parallely connected to each other, but the presentdisclosure is not limited thereto. Specifically, each switching elementS*# can be comprised of three or more switching elements parallelyconnected to each other, but the present disclosure is not limitedthereto.

If, for example, each switching element S*# is comprised of threeswitching elements parallely connected to each other, the drivecontroller 44 can be configured to execute, in step S42,

(1) Obtain the temperatures of the respective three switching elements

(2) Select the lowest temperature in the obtained temperatures

(3) Refer to the correlation data G1A to thereby select a value of thevariable for the gate determination voltage of each switching element atthe selected lowest temperature, thus commonly setting the selectedvalue as the gate determination voltage for each switching element

Note that each of the drive controllers 44C and 44D according to thethird and fourth embodiments can be configured to execute theovercurrent determination routine illustrated in FIG. 10 withoutexecuting the overcurrent determination routine illustrated in FIG. 3for each of the first and second switching elements parallely connectedto each other. This is because executing merely the overcurrentdetermination routine illustrated in FIG. 10 for the pair of first andsecond switching elements achieves a sufficient benefit. For thisreason, each of the drive controllers 44C and 44D according to the thirdand fourth embodiments can be configured to execute the overcurrentdetermination routine illustrated in FIG. 3 for each of the first andsecond switching elements parallely connected to each other, or not toexecute the overcurrent determination routine illustrated in FIG. 3 foreach of the first and second switching elements parallely connected toeach other.

When it is determined that an overcurrent is flowing through a switchingelement S*#, the drive controller is configured to set the resistancevalue of the gate discharging path to be higher than the resistancevalue of the normal gate discharging path, thus discharging the gate ofthe switching element S*# at a discharging rate lower than a normaldischarging rate in which it is determined that no overcurrent isflowing through the switching element S*# in step S24, but the presentdisclosure is not limited to this configuration.

As a first modification, from the structure of each drive unit DUillustrated in FIG. 2, the fourth terminal T4, soft turnoff resistor 38,and soft turnoff switching element 40 are eliminated. Then, a powersupply unit 50, which is comprised of a switching element, such as aMOSFET, and a constant current power source, is provided in each driveunit DU (see an imaginary block 50). The constant current power sourceis connected to the connection line between the third terminal T3 andthe discharging switching element 30.

Specifically, the drive controller 44 according to the firstmodification turns on the switching element of the power supply unit 50in step S24 to thereby supply electrical charge from the constantcurrent power source to the connection line between the third terminalT3 and the discharging switching element 30. This enables thedischarging rate of the gate of the switching element S*# during theoccurrence of an overflow through the switching element S*# to be slowerthan the discharging rate of the gate of the switching element S*#during no occurrence of an overflow through the switching element S*#.

As a second modification, from the structure of each drive unit DUillustrated in FIG. 2, the fourth terminal T4, soft turnoff resistor 38,and soft turnoff switching element 40 are eliminated.

Specifically, each drive unit DU according to the second modificationincludes a switching element, such as a MOSFET, 52 connected to thesource of the discharging switching element 30 and also connected to thedrive controller 44 (see an imaginary block 52). Each drive unit DUaccording to the second modification also includes a power source 53having a potential higher than the emitter of the switching element S*#(see an imaginary block 53).

Specifically, the drive controller 44 according to the secondmodification electrically turns on the switching element in step S24 tothereby cause the discharging rate of the gate of the switching elementS*# during the occurrence of an overflow through the switching elementS*# to be slower than the discharging rate of the gate of the switchingelement S*# during no occurrence of an overflow through the switchingelement S*#.

Each of the first to fourth embodiments is configured such that theconstant current power source 24 supplies electrical charge to theopen-close control terminal of each switching element S*# in a constantcurrent control mode, but the present disclosure is not limited thereto.Specifically, the constant current power source 24 can be eliminatedfrom each drive unit DU illustrated in FIG. 2, and the constant voltagepower source 22 can be connected to the first terminal T1. This causesthe constant voltage power source 22 to supply electrical charge to theopen-close control terminal of each switching element S*# in a constantvoltage control mode.

In each of the first to fourth embodiments, an IGBT is used as eachswitching element S*#, but the present disclosure is not limitedthereto. A MOSFET can be used as each switching element S*#.

In each of the first to fourth embodiments, the inverter 11 is used as aswitching circuit, but another type of switching circuits, such as afull bridge circuit, can be used.

While illustrative embodiments of the present disclosure have beendescribed herein, the present disclosure is not limited to theembodiment described herein, but includes any and all embodiments havingmodifications, omissions, combinations (e.g., of aspects across variousembodiments), adaptations and/or alternations as would be appreciated bythose in the art based on the present disclosure. The limitations in theclaims are to be interpreted broadly based on the language employed inthe claims and not limited to examples described in the presentspecification or during the prosecution of the application, whichexamples are to be construed as non-exclusive.

What is claimed is:
 1. An overcurrent determining apparatus applicableto a switching circuit that includes: a switching element having firstand second main terminals and an open-close control terminal; and acharging unit configured to supply electrical charge to the open-closecontrol terminal of the switching element to thereby charge theopen-close control terminal, the overcurrent determining apparatus beingconfigured to execute an overcurrent determination routine to determinewhether an overcurrent is flowing through the pair of first and secondmain terminals of the switching element based on whether a predeterminedcondition that a terminal voltage at the open-close control terminal ofthe switching element is higher than a determination voltage issatisfied, the overcurrent determining apparatus comprising: atemperature obtainer configured to obtain a temperature parameterindicative of a temperature of the switching element as a temperaturemeasurement value, the determination voltage having a first voltagevalue when the temperature measurement value is a first temperature; anda setter configured to set the determination voltage to a second voltagevalue upon determining that the temperature measurement value is asecond temperature which is higher than the first temperature, thesecond voltage value being lower than the first voltage value and higherthan a value of a Miller voltage of the switching element at the secondtemperature.
 2. The overcurrent determining apparatus according to claim1, wherein: the condition that the terminal voltage is higher than thedetermination voltage is defined as a first condition; the predeterminedcondition includes a second condition that an inter-terminal currentparameter indicative of an inter-terminal current flowing through thepair of first and second main terminals is higher than a current-relateddetermination value; and the second voltage value is set to be lowerthan a border voltage, the border voltage being a value of the terminalvoltage at a timing when the inter-terminal current parameter reachesthe current-related determination value when it is assumed that anovercurrent is flowing through the pair of first and second mainterminals of the switching element.
 3. The overcurrent determiningapparatus according to claim 2, further comprising: a voltage obtainerconfigured to obtain the terminal voltage; a current parameter obtainerconfigured to obtain the inter-terminal current parameter indicative ofthe inter-terminal current; and a determiner configured to: determine:whether the first condition that the terminal voltage obtained by thevoltage obtainer is higher than the determination voltage is satisfied;and whether the second condition that a value of the inter-terminalcurrent parameter is higher than the current-related determination valueis satisfied; and determine that an overcurrent is flowing through theswitching element upon determining that the first and second conditionsare satisfied.
 4. The overcurrent determining apparatus according toclaim 1, wherein: the switching element comprises at least first andsecond switching elements parallely connected to each other; thetemperature obtainer is configured to obtain, as the temperatureparameter, at least first and second temperature parameters respectivelyindicative of temperatures of the at least first and second switchingelements, the at least first and second temperature parameters beingrespectively referred to as at least first and second temperaturemeasurement values; and the setter has correlation data indicative of arelationship between the determination voltage and each of the at leastfirst and second temperature measurement values such that, the highereach of the at least first and second temperature measurement values is,the lower the determination voltage is, the setter being configured tocommonly set a value of the determination voltage for each of the firstand second switching elements to a selected value on the correlationdata, the selected value corresponding to the lowest value in the atleast first and second temperature measurement values.
 5. Theovercurrent determining apparatus according to claim 4, wherein: thecharging unit has a charging terminal for charging the open-closecontrol terminal of each of the first and second switching elements; andthe open-close control terminal of each of the first and secondswitching elements is connected to the charging terminal of the chargingunit, the overcurrent determining apparatus further comprising: avoltage obtainer configured to obtain a voltage at the charging terminalto thereby obtain the terminal voltage of each of the first and secondswitching elements.
 6. The overcurrent determining apparatus accordingto claim 5, further comprising: a current parameter obtainer configuredto obtain an inter-terminal current parameter indicative of theinter-terminal current for each of the first and second switchingelements; and a determiner configured to: determine, for at least one ofthe first and second switching elements, whether: a first condition,which is the condition, that the terminal voltage of the at least one ofthe first and second switching elements obtained by the voltage obtaineris higher than the determination voltage is satisfied; and a secondcondition that a value of the corresponding inter-terminal currentparameter of the at least one of the first and second switching elementsis higher than a current-related determination value is satisfied; anddetermine that an overcurrent is flowing through the at least one of thefirst and second switching elements upon determining that the first andsecond conditions are satisfied.
 7. The overcurrent determiningapparatus according to claim 1, wherein: the switching element comprisesat least first and second switching elements parallely connected to eachother; the temperature obtainer configured to obtain, as the temperatureparameter, a first temperature parameter indicative of a temperature ofthe first switching element as a first temperature measurement value,and a second temperature parameter indicative of a temperature of thesecond switching element as a second temperature measurement value; andthe setter has correlation data indicative of the relationship betweenthe determination voltage and each of the at least first and secondtemperature measurement values such that, the higher each of the atleast first and second temperature measurement values is, the lower thedetermination voltage is, the setter being configured to individuallyset a value of the determination voltage for each of the first andsecond switching elements to a selected value on the correlation data,the selected value for the first switching element corresponding to thefirst temperature measurement value, the selected value for the secondswitching element corresponding to the second temperature measurementvalue.
 8. The overcurrent determining apparatus according to claim 7,further comprising: a voltage obtainer configured to obtain a voltage atthe charging terminal to thereby obtain the terminal voltage of each ofthe first and second switching elements; a current parameter obtainerconfigured to obtain an inter-terminal current parameter indicative ofthe inter-terminal current for each of the first and second switchingelements; and a determiner configured to: determine, for at least one ofthe first and second switching elements, whether: a first condition,which is the condition, that the terminal voltage of the at least one ofthe first and second switching elements obtained by the voltage obtaineris higher than the determination voltage is satisfied; and a secondcondition that a value of the corresponding inter-terminal currentparameter of the at least one of the first and second switching elementsis higher than a current-related determination value is satisfied; anddetermine that an overcurrent is flowing through the at least one of thefirst and second switching elements upon determining that the first andsecond conditions are satisfied.
 9. The overcurrent determiningapparatus according to claim 3, further comprising: a normal turnoffunit configured to discharge the open-close control terminal of theswitching element at a first discharging rate upon it being determinedthat no overcurrent is flowing through the switching element; and a softturnoff unit configured to discharge the open-close control terminal ofthe switching element at a second discharging rate to thereby forciblyturn off the switching element upon it being determined that anovercurrent is flowing through the switching element, the seconddischarging rate being lower than the first discharging rate.
 10. Anovercurrent determining apparatus applicable to a switching circuit thatincludes: at least first and second switching elements parallelyconnected to each other, each of the at least first and second switchingelements having first and second main terminals and an open-closecontrol terminal; and a charging unit configured to supply electricalcharge to the open-close control terminal of each of the at least firstand second switching elements to thereby charge the open-close controlterminal, the overcurrent determining apparatus being configured toexecute an overcurrent determination routine to determine whether anovercurrent is flowing through the pair of first and second mainterminals of each of the at least first and second switching elementsbased on whether a predetermined condition that a terminal voltage atthe open-close control terminal of the corresponding one of the at leastfirst and second switching elements is higher than a determinationvoltage is satisfied, the overcurrent determining apparatus comprising:a temperature obtainer configured to obtain at least first and secondtemperature parameters respectively indicative of temperatures of the atleast first and second switching elements, the at least first and secondtemperature parameters being respectively referred to as at least firstand second temperature measurement values; and a setter havingcorrelation data indicative of a relationship between the determinationvoltage and each of the at least first and second temperaturemeasurement values such that, the higher each of the at least first andsecond temperature measurement values is, the lower the determinationvoltage is, the setter being configured to commonly set a value of thedetermination voltage for each of the at least first and secondswitching elements to a selected value on the correlation data, theselected value corresponding to the lowest value in the at least firstand second temperature measurement values.
 11. The overcurrentdetermining apparatus according to claim 10, wherein: the charging unithas a charging terminal for charging the open-close control terminal ofeach of the at least first and second switching elements; and theopen-close control terminal of each of the at least first and secondswitching elements is connected to the charging terminal of the chargingunit, the overcurrent determining apparatus further comprising: avoltage obtainer configured to obtain a voltage at the charging terminalto thereby obtain the terminal voltage of each of the at least first andsecond switching elements.
 12. The overcurrent determining apparatusaccording to claim 11, further comprising: a current parameter obtainerconfigured to obtain an inter-terminal current parameter indicative ofthe inter-terminal current for each of the at least first and secondswitching elements; and a determiner configured to: determine, for atleast one of the at least first and second switching elements, whether:a first condition, which is the condition, that the terminal voltage ofthe at least one of the at least first and second switching elementsobtained by the voltage obtainer is higher than the determinationvoltage is satisfied; and a second condition that a value of thecorresponding inter-terminal current parameter of the at least one ofthe first and second switching elements is higher than a current-relateddetermination value is satisfied; and determine that an overcurrent isflowing through the at least one of the at least first and secondswitching elements upon determining that the first and second conditionsare satisfied.
 13. The overcurrent determining apparatus according toclaim 12, further comprising: a normal turnoff unit configured todischarge the open-close control terminal of each of the first andsecond switching elements at a first discharging rate upon it beingdetermined that no overcurrent is flowing through each of the at leastfirst and second switching elements; and a soft turnoff unit configuredto discharge the open-close control terminal of the at least one of theat least first and second switching elements at a second dischargingrate to thereby forcibly turn off the at least one of the at least firstand second switching elements upon it being determined that anovercurrent is flowing through the at least one of the at least firstand second switching elements, the second discharging rate being lowerthan the first discharging rate.
 14. A drive unit comprising: aswitching circuit that includes: a switching element having first andsecond main terminals and an open-close control terminal; and a chargingunit configured to supply electrical charge to the open-close controlterminal of the switching element to thereby charge the open-closecontrol terminal; and a drive controller configured to: execute anovercurrent determination routine to determine whether an overcurrent isflowing through the pair of first and second main terminals of theswitching element based on whether a predetermined condition that aterminal voltage at the open-close control terminal of the switchingelement is higher than a determination voltage is satisfied; obtain atemperature parameter indicative of a temperature of the switchingelement as a temperature measurement value, the determination voltagehaving a first voltage value when the temperature measurement value is afirst temperature; and set the determination voltage to a second voltagevalue upon determining that the temperature measurement value is asecond temperature higher than the first temperature, the second voltagevalue being lower than the first voltage value and higher than a valueof a Miller voltage of the switching element at the second temperature.